If you opened a new Private Browsing window, it is active.

In addition to the above :

It wll be explained in this article (you'll have to scroll down) :

https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history

In the previous version, a Mask would appear at the top of the screen. It no longer is visible... prompting my question.... How do I know I am in Private Browsing mode?

In that article I gave you the link to, did you scroll down to the part that
says :

"Important: When Firefox is set to Never remember history you won't see a purple mask at the top of each window, even though you are effectively in Private Browsing mode. To restore normal browsing, go to your Privacy Options and set Firefox to Remember history. "